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RocketmanOU
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« on: February 14, 2012, 08:58:59 PM »

I profiled a part this morning that should be relatively simple, but the arcs are coming out as what seem to be either line segments or much smaller arcs. I don't see anything in the g-code that should cause this, so I'm pretty confident it's not a Cam issue (I'm using CamBam). Any ideas what might be causing this and how I could correct it? Thanks!








G-Code:

G20 G90 G91.1 G64 G40
G0 Z0.5
T5 M6
( Outside Profile)
G17
M3 S1000
G0 X-6.1827 Y-0.4475
G1 F10.0 Z-0.03
G1 F30.0 X-1.4882
G2 X-0.6681 Y-0.7575 I0.0 J-1.24
G3 Y0.7575 I0.6681 J0.7575
G2 X-1.4882 Y0.4475 I-0.8202 J0.93
[...repeated again and again and again with decreasing z...]
G0 Z0.5
M5
M30
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« Reply #1 on: February 14, 2012, 10:42:12 PM »

First thought is, what is the resolution of the machine ?
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RocketmanOU
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« Reply #2 on: February 14, 2012, 11:14:04 PM »

Quantified how? My motors are 10000 pulses per inch, running Mach3 at 45 kHz kernel frequency on a very powerful  (3.2 GHz quad core, 4 GB RAM) computer. I have glass scales on the axes so I can watch real measurements, and I can get precise motion down to about 0.001" or so when I'm using backlash compensation. Backlash in the machine is only 0.004" or so anyway though. For the record, I cut the rough cut with backlash compensation on, then did the final pass with it off to see if it made any difference - nada.
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« Reply #3 on: February 14, 2012, 11:23:20 PM »

The electronic resolution at .0001 is fine enough, but .004 backlash is huge.
Have you had better result with arcs prior to these ?
What type of machine is it ? Router ? Mill ?, ? ? ?
The extent of y in the toolpath is about 1.75", what is the dia. of the tool ? or, what is the rad. of the rod ends ?
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RICH
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« Reply #4 on: February 15, 2012, 06:22:42 AM »

The straights along the arc are rather consistant. Would be interesting to have you measure them , then draw the flat inside  or along the diameter
and see how much deviation there is from the arc. I am going to guess it's related to the backlash and the deviation from the arc is close  or relates to the
backlash.  Whats the different in measurement across flats as compared to arc?

Not sure how the BC  is implemented during CV mode. Huh
May be all wet......... Wink

RICH
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RocketmanOU
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« Reply #5 on: February 15, 2012, 08:02:30 AM »

The outside arc radii are 0.75" and 0.375", and the inside radii (the fillets) are 1.5". I used a 1/2" endmill for roughing and a 1/4" stubby endmill with large shank for the finish pass. It's a LMS mini mill with the CNCFusion ballscrew package. These are some of the first parts coming off the machine, so I can't speak to whether this is something that just cropped up or not.

The length of the flats are .160" on the outside of the large diameter (0.75") and 0.120" on the outside of the smaller diameter. They're also not flat, now that I take a close look. They're definitely arcs in and of themselves.

If this were due to backlash, I would expect to see a seam any place an axis changes direction, but only there. In other words, I should see variation at these points, and these points alone:



Outside of the backlash, the axes are very consistent - example: I make a g0 move from x0 to x-3.7500, then from x-3.7500 to x-3.5000. I now zero the glass DRO, and I can make any move in the positive direction and be pretty much dead on (within a couple of ten thousandths, anyway).
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« Reply #6 on: February 15, 2012, 08:29:40 AM »

You might try CV/ES with BC On/Off just to see if there is a difference.
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RocketmanOU
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« Reply #7 on: February 15, 2012, 08:49:33 AM »

As I said, I tried CV with BC on and off, and there was no difference. I can give exact stop a try if you think it would help.
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Dan13
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« Reply #8 on: February 15, 2012, 10:30:18 AM »

Doesn't look like backlash to me. I would, however, guess that it is a mechanical problem in one (or both) of the axes. If you have belt transmission, for instance, a run-out of the pulley could cause something like this.

To make troubleshooting easier you could hold a pencil in the spindle and draw the tool path on a paper.

By the way, the transition points you specified in the above drawing are not all correct. The 2 diagonal lines should be removed as there is no direction change in either axis.

Dan
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RocketmanOU
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« Reply #9 on: February 15, 2012, 10:53:02 AM »

Doesn't look like backlash to me. I would, however, guess that it is a mechanical problem in one (or both) of the axes. If you have belt transmission, for instance, a run-out of the pulley could cause something like this.


My motors are direct drive, and the Lovejoy couplers are nice and snug. Plus, if it were a problem such as that, I'd also see ridges (or at least feed changes) on long, straight sections, and I'm not seeing that at all. Per part inspection, the feed on the straight sections is remarkably uniform.

By the way, the transition points you specified in the above drawing are not all correct. The 2 diagonal lines should be removed as there is no direction change in either axis.

Ah, good point. I was thinking in velocity and not displacement.
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